1. Field of the Invention
The present invention relates to a heat transfer member, and more particularly, to an external heat transfer member and a transition member having improved coupling strength.
2. Description of the Related Art
Generally, a variety of reciprocating devices, including but not limited to free-piston machines, are often used in a heat regeneration type of refrigerator, including but not limited to Stirling coolers, Gifford-McMahon refrigerators, and the like.
A conventional free-piston machine is described in U.S. Pat. No. 6,293,184, which issued to Unger on Sep. 25, 2001, the contents of which are expressly incorporated by reference in its entirety. Additionally, hereinafter, the structure and operation of a conventional typical free piston machine is described in FIG. 1, which shows a sectional view of a typical free-piston machine.
The free-piston machine includes a sealing container 10, a cylinder 20 installed in the inside of the sealing container 10, for receiving a gas therein, a piston 22 mounted inside of the cylinder 20, a displacer housing 30 provided on one side of the cylinder 20, a displacer 32 movably installed inside the displacer housing 30, for compressing and expanding a gas while moving in combination with the piston 22, a regenerator 40 for absorbing thermal energy from the gas and storing/radiating the thermal energy, and a linear motor 50 for driving the piston 22.
The displacer 32 is configured to have a displacer rod 321 on its one end, which penetrates the piston 22 and is supported by a planar spring 12 on the lower side of the cylinder 20. The planar spring 12 linearly reciprocates within its range of elastic deformation. The displacer 32 is configured to also include the regenerator 40 therein.
A compression space 30a is provided between the piston 22 and the displacer 32, for compressing a gas by the combined movement of the piston 22 and the displacer 32. An expansion space 30b is provided on the front inner side of a finger tube 14, for expanding a gas.
The free-piston machine also includes a heat transfer member for gradually reducing the energy level of the gas in a cycle including the compression space 30a and the expansion space 30b, and the regenerator 40 therebetween. In detail, the heat transfer member includes internal/external heat transfer members 17, 18 respectively internally and externally mounted on the transition member 16 which connects a finger tube 14 with the sealing container 10.
Referring to FIGS. 1 and 2, the internal heat transfer member 17 includes a base 171 having a generally tubular shape and attached to the inside wall of the transition member 16, and a plurality of heat-absorbing fins 172 protruding inwardly from the base 171. The external heat transfer member 18 includes a base 181 having a generally tubular shape and adhesively attached with the outer side wall of the transition member 16, and a plurality of heat-absorbing fins 182 protruding outwardly from the base 181.
The base 181 of the external heat transfer member 18 is made bigger in volume than the base 171 of the internal heat transfer member 17 so as to increase the heat transfer effect. In addition, there is provided an air pocket (18a in FIG. 3), which is an empty space between the transition member 16 and the external heat transfer member 18, and which does not overlap with the internal heat transfer member 17 and is bigger in diameter than the internal heat transfer member 17.
While the gas compressed in the compression space 30a passes through the transition member 16 prior to being introduced into the regenerator 40, it makes contact with the internal heat transfer member 17 and conducts its thermal energy out of the transition member 16 through the external heat transfer member 18. Therefore, the energy level of the gas is gradually lowered, and unnecessary energy loss can be prevented due to the presence of the air pocket 18a beyond the location of the internal heat transfer member 17 because the continuous transferring of the heat is stopped.
In order to improve sealing capabilities between respective components during manufacture of the free-piston machine, the front end of the transition member 16, the internal and external heat transfer members 17, 18 and an adaptor ring 19 are coupled by brazing.
Referring to FIG. 3, in the brazing process, a ring-shaped brazing material P is applied to the front end of the external heat transfer member 18, and an induction coil C is mounted on the adaptor ring 19. Then, power is applied to the induction coil C, and each component is heated to melt the brazing material P.
However, in the conventional art, since the transition member 16 and the adaptor ring 19 are made of stainless steel, and the external heat transfer member 18 is made of copper, the melted brazing material P mostly flows toward the external heat transfer member 18, which has a relatively high thermal conductivity because of its material property (i.e. copper). Therefore, the brazing portion of the transition member 16 and the adaptor ring 19 may have an unreasonably weak strength.
In addition, because the base 181 of the external heat transfer member 18 and the transition member 16 have a narrow clearance (about 50 μm), the brazing material P does not flow through the clearance between the adaptor ring 19 and the external heat transfer member 18, and also does not flow through the clearance between the transition member 16 and the external heat transfer member 18. Therefore, the brazing strength between the external heat transfer member 18 and the transition member 16, and the brazing strength between the external heat transfer member 18 and the adaptor ring 19 is not uniform, thereby potentially causing problems with the braze.
Furthermore, in the brazing process, the air is heated inside the air pocket 18a and expanded to be introduced into the clearance between the external heat transfer member 18 and the transition member 16 such that the air bubbles are generated in the melted brazing material P, thereby reducing the sealing capabilities thereof.